A CMOS self-calibrating frequency synthesizer

A programmable phase-locked-loop (PLL)-based frequency synthesizer, capable of automatically adjusting the nominal center frequency of the voltage-controlled oscillator (VCO) to an optimum value is described, In fully integrated PLLs, the VCO output frequency should be tunable over a wide range of frequencies, covering the desired range of the synthesizer output frequencies, for all processing variations and operating conditions. A wide tuning range realized by making the VCO gain K-o large has the unwanted effect of increasing the phase noise at the output of the VCO, and hence the PLL as well, In this work, the wide tuning range is realized by digital control, with process variability managed through self-calibration. The PLL is only required to pull the oscillator output frequency to account for the digital quantization, temperature variations, and some margin. This allows the K-o to be small, with better noise performance resulting. The prototype self-calibrating frequency synthesizer, capable of operating from 80 MHz to 1 GHz, demonstrates a measured absolute jitter of 20-ps rms at 480-MHz operating frequency. The prototype IC is fabricated in a 0.35-mu m 3-V digital CMOS process.